1. Field of the Invention
This invention relates generally to detection of special nuclear materials, and, more specifically, to a compact, low-cost gamma ray generator to aid in such detection.
2. Background
Many non-intrusive active interrogation techniques utilize neutrons or gamma rays to detect special nuclear material (SNM) concealed in cargo. For active interrogation systems with neutron sources, neutron induced gamma rays are detected and, sometimes, transmitted neutrons are measured. Neutron induced gamma spectra of different materials are used as the fingerprints for them. Fast neutrons are often in use to obtain a deep penetration into large inspected objects and, thus, generate a very high background from surrounding materials. While this high background restricts the maximum screening speed of many neutron-based systems, neutrons also tend to activate the surrounding materials after an extensive long period of operation.
On the other hand, gamma-based systems detect neutrons produced from photonuclear reactions or transmitted gamma rays. Because the neutron production cross sections of many special nuclear materials due to photofission are much higher than that of most common materials, the neutron background in gamma-based interrogation techniques is fairly low. Furthermore, the induced radioactivity of surrounding materials due to gamma rays of less than 16 MeV is rather small due to the high threshold energy of photonuclear reactions. However, most existing gamma-based interrogation systems use electron linacs and microtrons to generate the gamma beams; thus, the deployment of these systems is limited by their size, complexity and high cost of ownership. Thus there is a need for low-cost, portable gamma sources to use in active interrogation systems to detect SNM.